Cisco CCNA Study Guide - Router Alley

CCNA Study Guide v2.62 – Aaron Balchunas
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Native VLANs
A native VLAN can also be configured on trunk ports:
Switch(config)# interface fa0/24
Switch(config-if)# switchport mode trunk
Switch(config-if)# switchport trunk native vlan 42
Frames from the native VLAN are not tagged when sent out trunk ports. A
trunking interface can only be assigned one native VLAN. Only 802.1Q
supports native VLANs, whereas ISL does not. (More accurately, ISL will
tag frames from all VLANs, even if a VLAN is configured as native). The
native VLAN should be configured identically on both sides of the 802.1Q
trunk).
Native VLANs are often configured when plugging Cisco VoIP phones into
a Catalyst Switch (beyond the scope of this section). Native VLANs are also
useful if a trunk port fails. For example, if an end user connects a computer
into a trunk port, the trunking status will fail and the interface will
essentially become an access port. The user’s computer will then be
transparently joined to the Native VLAN.
Native VLANs provide another benefit. A trunk port will accept untagged
frames and place them in the Native VLAN. Consider the following
example:
Assume that both 802.1Q switches have trunk links configured to the non-
802.1Q switch, and that the trunk ports are configured in Native VLAN 42.
Not only will the 802.1Q switches be able to communicate with each other,
the non-802.1Q switch will be placed in Native VLAN 42, and be able to
communicate with any device in VLAN 42 on any switch.
(Please note, that the author of this study guide finds the “benefit” of the
above example of Native VLANs to be……dubious at best, and confusing
as hell at worst).
By default on all trunking interfaces, the Native VLAN is VLAN 1.
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All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com),
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